How can I access history list? (python)
Unfortunately, you cannot access the history list directly from python (I'm pretty confident this is the case.).

The history list is called by bpy.ops.ed.undo_history() and doesn't have any accessible attributes that you can read into a list. In fact, bpy.ops.ed's methods come from bpy.types where the class  starts with ED_OT_ (eg. bpy.ops.ed.undo() is really an instance of bpy.types.ED_OT_undo()

It looks like the undo stuff is in the guts of blender-app.exe but you can try and mess around with this bpy.ops.ed stuff and see if you can manage something that I may be overlooking.